Stator for an electrical machine

ABSTRACT

( 57 ) A stator ( 50 ) for an electrical machine. A core of the stator ( 50 ) is made up of a series of connected segments ( 42 ), that are rotatable relative to one another during an assembly operation. Each segment ( 42 ) includes a radially outermost base portion ( 44 ) from which a respective integral pole piece ( 46 ) projects substantially inwardly to a pole piece tip. The tip includes a surface ( 49 ) that is arcuate and concentric with an axis of rotation of the machine. Each pole piece ( 46 ) projects in a non-radial direction and such that a semi-closed slot ( 48 ) arranged to locate part of a least one coil ( 30 ) therein is provided between each pair of juxtaposed pole pieces ( 46 ). A coil ( 30 ) is fittable to a respective one of the pole pieces ( 46 ) during the assembly operation without deformation of the pole piece ( 46 ).

[0001] The present invention relates to stators for electrical machines.

[0002] Electrical machines such as motors, dynamos or alternatorsgenerally include a stator having a laminated metal core with a numberof coils supported thereon in an annular arrangement. A rotor, also withcoils supported thereon, is commonly positioned within the annulus andmounted for rotation therein. The rotor and stator are separated by asmall, annular air gap.

[0003] Stator constructions are known in which electromagnetic coils arewound in situ in slots in the stator core, or are forced into the slotsthrough mouths of the slots. Such slots tend to have a narrow mouthformed in a radially inner surface of the stator core that leads to alarger cavity. Frequently the slots resemble “tear-drop” shapes incross-section and are referred to as “semi-closed” slots due to thenarrow mouths. As is known, a narrow mouth of each slot is advantageousin maximising flux in the air gap, in reducing “cogging” and inimproving the quality of the sine wave output of alternators.

[0004] However, inserting conductors of stator coils into semi-closedslots is difficult.

[0005] This is a problem in itself, but also results in the proportionof slot space occupied by conductors of the coils being fairly small(typically 35%). Air occupies the remainder of the slots and acts as aninsulant that hinders cooling of the conductors.

[0006] EP-A-0871282 discloses a further stator arrangement that is shownin FIG. 1. This stator includes an annular stator core 12 formed bystacking a number of steel sheets on top of each other. Each steel sheetincludes a plurality of unit cores 13, each of which includes a unityoke 14 and a magnetic pole tooth 15. The unit cores 13 are sequentiallyconnected together by connecting portions 13 a. Each magnetic pole tooth15 includes a stator coil 18 thereon. The coils are held in position bymeans of protrusions 16 at the end of each pole tooth 15 furthermostfrom the respective yoke 14. The protrusions 18 are plasticallydeformable to provide an end stop to located the coil 18 on therespective tooth 15.

[0007] The stator disclosed in EP-A-0871282 has several disadvantagesassociated therewith. Firstly, plastic deformation of the protrusions 16alters the electromagnetic properties of the electrical steel of theprotrusions 16, leading to reduced permeability thereof and greater ironlosses. In addition, plastic deformation causes the protrusions 16 totend to lack concentricity with the axis of rotation, leading to anundesirably large and irregular air gap between the protrusions 16 andan adjacent rotor. This increases the reluctance of the air gap andreduces the performance of the machine.

[0008] It is an object of this invention to address one or more of thesedisadvantages.

[0009] According to an aspect of this invention there is provided amethod of manufacturing a stator for an electrical machine, the methodincluding the steps of:

[0010] a) selecting a plurality of core segments which areinterconnected to form a strip, each core segment including asubstantially arcuate base portion which has a convex side, a concaveside and a pair of ends and a pole piece which projects from the concaveside and from between the ends of the base portion, at least one of theends of each core segment being connected to an end of an adjacent coresegment and each pole piece having a tip which forms an arcuate surface;

[0011] b) fitting a respective pre-wound coil to at least one of thepole pieces; and

[0012] c) forming said strip of core segments with said at least onecore fitted thereon into an annular stator such that the arcuatesurfaces at the pole piece tips are substantially concentric,characterised in that each pole projects from the concave side of thesubstantially arcuate base portion in a non-radial direction and asemi-closed slot is formed between it and an adjacent one of the polepieces whereby said at least one coil is located between the respectivepole piece and the adjacent pole piece.

[0013] According to another aspect of this invention there is provided acomponent for forming into an annular stator core of an electricalmachine, the component being a plurality of core segments interconnectedto form a strip, each core segment including a substantially arcuatebase portion which has a convex side, a concave side and a pair of endsand a pole piece which projects from the concave side and from betweenthe ends of the base portion, at least one of the ends of each coresegment being connected to an end of an adjacent core segment and eachpole piece having a tip that forms an arcuate surface, wherein each polepiece is adapted to receive at least one coil thereon and said strip ofcore segments is adapted to be formed, subsequent to receiving at leastone coil on a respective one of the pole pieces, into an annular statorcore, wherein the arcuate surfaces formed by the pole pieces aresubstantially concentric, characterised in that each pole piece projectsfrom the concave side of the base portion in a non-radial direction andis arranged such that a semi-closed slot is formed between it and anadjacent one of the pole pieces whereby, when said at least one coil isfitted onto the respective pole piece and said strip of core segments isformed into the annular stator core, said at least one coil is locatedbetween the respective pole piece and the adjacent pole piece.

[0014] Preferably each pole piece projects in a direction that issubstantially oblique to a radius therethrough.

[0015] Preferably, the perpendicular distance between sides of each polepiece is the same or smaller towards the respective tip than towards therespective base portion, thereby allowing a coil to be moved over thetip of the pole piece and to be fitted in position around the respectivepole piece. Preferably inner surfaces of the coil are in mating contactwith the sides of the respective pole piece, when the pole piece isfitted thereon. Preferably the perpendicular distance between the sidesof each pole piece is constant for the length of each pole piece alongwhich the respective coil is moved during fitting. The sides of eachpole piece may be arcuate, and preferably are concentric arcs, for thelength of each pole piece along which the respective coil is movedduring placement. Preferably the arcs curve away from a radius through aroot of the respective pole piece, the root being adjacent therespective base portion.

[0016] The sides of the pole piece may be: straight, parallel sides;straight sides that converge as they project from the base portion; orarcuate sides, the perpendicular distance between which reduces as theyproject from the base portion.

[0017] It will be appreciated that providing a pole piece that projectsother than radially and for which the perpendicular distance between thesides is constant for the length of each pole piece along which therespective coil is moved during placement results in the arcuate surfaceat the tip of that pole piece being of greater curved length than wouldbe the case if the pole piece projected radially with the sameperpendicular distance between the sides of the pole piece. This isadvantageous in reducing the gap between juxtaposed pole piece tips,while maintaining a cavity between the respective pole pieces toaccommodate coil portions, i.e. in optimising the shape of thesemi-closed slots.

[0018] Each segment of the component is rotatable relative to the othersegments thereof from an assembly position in which the segments lie ina substantially linear series to an operational position in which thesegments form the annular stator core.

[0019] Preferably each base portion of each segment is attached to abase portion of at least one other segment by a thin strip of integralmaterial adjacent a radially outermost surface of each base portion.

[0020] Preferably, each coil is a pre-formed coil, such as a pre-wound“bobbin” of wire turns. Each coil may be mounted on a carrier.Preferably the shape of each coil is such that when it fitted to therespective pole piece and the strip of core segments is formed into thestator, the cavities are substantially filled thereby.

[0021] The strip of core segments may include a plurality of stackedlaminations. The strip of segments may include a respective coil onevery other pole piece.

[0022] Specific embodiments of this invention are now described by wayof example only and with reference to the accompanying, drawings, inwhich:

[0023]FIG. 1 is a plan view of a known stator;

[0024]FIG. 2 is a plan view of a lamination for one stator in which theinvention is embodied;

[0025]FIG. 3 is a plan view of a part of the lamination;

[0026]FIG. 4 is a plan view of another part of the lamination and twocoils;

[0027]FIG. 5 is an axial view of the one stator; and

[0028]FIG. 6 is a plan view of an alternative stator in which theinvention is embodied.

[0029]FIG. 2 shows a lamination 20 for use in manufacturing a stator ofan electrical machine (not shown). It is envisaged that the electricalmachine may be an AC or DC motor or generator. The lamination 20 is athin strip of electrical steel that is stamped from a sheet of suchsteel (not shown). In an as-stamped arrangement, the lamination 20includes a linear series of segments 22. In this embodiment there arenine such segments 22, but it is envisaged that any number of segments22 may be used. Each segment 22 includes a base portion 24 that issubstantially arcuate and an integral pole piece 26 that projects from aradially inner edge of the base portion 24. Each segment 22 is joined toeach of two respective juxtaposed segments 22, apart from the twosegments 22 at each end of the series which are each joined to only onerespective segment 22. Juxtaposed segments 22 are joined by an integral,thin strip of material 27 between the respective base portions 24,adjacent a radially outer edge of the base portions 24.

[0030] For simplicity of description, a representative one of thesegments 22 will now be considered, with reference to FIG. 3. As stated,the pole piece 26 projects from the radially inner edge of the baseportion 24. However, the pole piece 26 projects in a direction otherthan along a radius of the arcuate base portion 24. In this respect, thesegment 22 resembles an eccentric T-shape, with the base portion 24 ofthe segment corresponding to the horizontal line of the “T” and the polepiece 26 corresponding to the vertical line of the “T”, albeit displacedto one or other side of the vertical. The pole piece 26 has a first anda second side 23,25 running the length of the pole piece 26 in thedirection of projection thereof. In this embodiment the two sides 23,25are arcuate and concentric.

[0031] The pole piece 26 has a tip that is remote from the base portion24. The tip includes an arcuate surface 29 that is concentric with thearcuate base portion 24.

[0032] In an assembly operation, a plurality of the laminations 20 areplaced on top of each other to form a stack of constant cross section,the cross section being the same as the plan form of each lamination 20.A part of the stack 40 is depicted end-on in FIG. 4. The number oflaminations 20 is that which is required to achieve the desired axiallength of the stator of the electrical machine, the height of the stack40 corresponding to the axial length of the stator. The laminations 20may be fixed relative to one another by a fixing operation such asgluing or welding.

[0033] When stacked, the segments 22 of each laminate 20 go to make upnine elongate segments 42 of the stack 40. The pole pieces 26 of eachsegment 22 go to make up nine elongate pole pieces 46 of the stack 40.Similarly the base portions 24 of each segment go to make up nineelongate base portions 44 of the stack 40.

[0034] Each elongate pole piece 46 is adapted to have a respective coilfitted therearound. Each coil is a pre-wound bobbin of predeterminedshape and is for use as a stator winding in the electrical machine. Twosuch coils 28,30 that are representative of all the coils are shown inFIG. 4 in cross section, the section being co-planar with thelaminations 20. In cross section, each coil 28,30 consists of two spacedapart coil portions, each coil portion having an inside surface that isopposite the other coil portion. A first one of the coils 28 is shownfitted in place around a first one of the elongate pole pieces 46 of thestack 40. The inside surfaces of the coils 28,30 are such that eachinside surface mirrors a respective one of the sides 23,25 of each polepiece 46, i.e. each inside surface is a co-axial arc. Thus when thecoils 28,30 are in a fitted position, as is the first coil 28 in FIG. 4,the inside surfaces are a close, mating fit against the sides 23,25 ofthe elongate pole piece 46.

[0035]FIG. 4 also shows a second one of the coils 30 being fitted to asecond one of the elongate pole pieces 46. As the sides 23,25 of thepole piece, and the inside surfaces of the coil 30 are concentric, thecoil 30 can be fitted by being slid into the fitted position along thesides 23,25 of elongate pole piece 46. FIG. 4 shows the coil 30 at twoinstants during the fitting thereof to the elongate pole piece 46. Arespective coil is fitted to each of the elongate pole pieces 46 of thestack 40 in this way.

[0036] As stated, in this embodiment the two sides 23,25 are arcuate andconcentric. However, it is envisaged that the sides 23,25 may be:straight, parallel sides, straight sides that converge as they projectfrom the respective base portion 24; or arcuate sides, the perpendiculardistance between which reduces as they project from the base portion 24.Almost any relationship between the sides may be used, provided that theresulting geometry permits the fitting of a coil to the respective polepiece 26. However, it is desirable, for electromagnetic reasons, for theinside surfaces of the coil to closely fit the sides 23,25 of the polepiece 26 when the pole piece 26 is in the fitted position. For thisreason, parallel sides 23,25 that diverge as they project from the baseportion 24; and arcuate sides, the perpendicular distance between whichincreases as they project from the base portion 24, are not favoured.

[0037] Following fitting of the coils to the elongate pole pieces 46,each elongate segment 42 of the stack 40 is rotated relative to eachother elongate segment 42 such that the stack 40 closes up to form anannular stator 50. The annular stator 50 is shown in FIG. 5. The annularstator 50 is adapted to receive a rotor (not shown) therein centred onan axis of rotation that is concentric with the arcuate surface 49 ofthe tip of each pole piece 46 and preferably with the arcuate baseportion 44 of each segment 42. The stack 40 may be fixed in thisclosed-up position by a further fixing operation such as gluing orwelding.

[0038] In the closed-up arrangement, the stator 50 includes nineelongate pole pieces 46, each with a radially innermost elongate surface49 that is arcuate and concentric with the axis of rotation of theelectrical machine. Concentricity of the elongate surfaces 49 isadvantageous in providing a uniform annular air gap between the statorand the rotor.

[0039] There is a small gap 38 between juxtaposed elongate surfaces 49.Each small gap 38 leads to a respective larger cavity 48 in body of thestator 50. Together each gap 38 and the respective cavity 48 constitutea semi-closed slot and are therefore electromagnetically advantageous.Each cavity 48 is substantially filled by a coil portion of each of twocoils.

[0040] From FIG. 5 it is evident that, by projecting in a non-radialdirection, each pole piece 46 is able to include at the tip thereof, anarcuate surface 49 that is of greater length than that which wouldresult if each pole piece 46 projected radially. The provision ofarcuate sides 23,25, for each pole piece 46, in preference to straightsides, is advantageous in increasing the cross-sectional area or eachcavity 48.

[0041]FIG. 6 shows a similar, but alternative stator 100. Pole pieces126 of the alternative stator 100 also have arcuate sides 123,125.However, the arcs are of smaller diameter. This leads to each cavity 148having a substantially circular cross-section, which in turn allows eachcoil portion fitted therein to be substantially semi-circular in crosssection. Coil portions of the same coil may therefore be symmetrical andbe more convenient to manufacture.

1. A method of manufacturing a stator for an electrical machine, themethod including the steps of: a) selecting a plurality of core segmentswhich are interconnected to form a strip, each core segment including asubstantially arcuate base portion which has a convex side, a concaveside and a pair of ends and a pole piece which projects from the concaveside and from between the ends of the base portion, at least one of theends of each core segment being connected to an end of an adjacent coresegment and each pole piece having a tip which forms an arcuate surface;b) fitting a respective pre-wound coil to at least one of the polepieces; and c) forming said strip of core segments with said at leastone core fitted thereon into an annular stator such that the arcuatesurfaces at the pole piece tips are substantially concentric,characterised in that each pole projects from the concave side of thesubstantially arcuate base portion in a non-radial direction and asemi-closed slot is formed between it and an adjacent one of the polepieces whereby said at least one coil is located between the respectivepole piece and the adjacent pole piece.
 2. A component for forming intoan annular stator core of an electrical machine, the component being aplurality of core segments interconnected to form a strip, each coresegment including a substantially arcuate base portion which has aconvex side, a concave side and a pair of ends and a pole piece whichprojects from the concave side and from between the ends of the baseportion, at least one of the ends of each core segment being connectedto an end of an adjacent core segment and each pole piece having a tipthat forms an arcuate surface, wherein each pole piece is adapted toreceive at least one coil thereon and said strip of core segments isadapted to be formed, subsequent to receiving at least one coil on arespective one of the pole pieces, into an annular stator core, whereinthe arcuate surfaces formed by the pole pieces are substantiallyconcentric, characterised in that each pole piece projects from theconcave side of the base portion in a non-radial direction and isarranged such that a semi-closed slot is formed between it and anadjacent one of the pole pieces whereby, when said at least one coil isfitted onto the respective pole piece and said strip of core segments isformed into the annular stator core, said at least one coil is locatedbetween the respective pole piece and the adjacent pole piece.
 3. Amethod according to claim 1 or a component according to claim 2 whereineach pole piece projects in a direction that is substantially oblique toa radius therethrough.
 4. A method according to claim 1 or claim 3 or acomponent according to claim 2 or claim 3, wherein the perpendiculardistance between sides of each pole piece is the same or smaller towardsthe respective tip than towards the respective base portion, therebyallowing a coil to be moved over the tip of tile pole piece and to befitted in position around the respective pole piece.
 5. A method or acomponent according to claim 4, wherein inner surfaces of the coil arein mating contact with the sides of the respective pole piece, when thepole piece is fitted thereon.
 6. A method or a component according toclaim 4 or claim 5, wherein, the perpendicular distance between thesides of each pole piece is constant at least for the length of eachpole piece along which the respective coil is moved during fitting.
 7. Amethod or a component according to any one of claims 4 to 6 wherein, thesides of each pole piece are arcuate, at least for the length of eachpole piece along which the respective coil is moved during fitting.
 8. Amethod or a component according to claim 7 wherein the sides of eachpole piece are concentric arcs, at least for the length of each polepiece along which the respective coil is moved during fitting.
 9. Amethod or a component according to any one of claims 4 to 6, wherein thesides of the pole piece are: straight, parallel sides; straight sidesthat converge as they project from the base portion; or arcuate sides,the perpendicular distance between which reduces as they project fromthe respective base portion.
 10. A method or a component according toclaim 7 or claim 8 wherein the arcuate sides curve away from a radiusthrough a root of the respective pole piece, the root being adjacent therespective base portion.
 11. A method or a component according to anyone of the preceding claims wherein the shape of each coil is such thatwhen it fitted to the respect pole piece, and the stator is formed, thecavities are substantially filled thereby.